Diode laser structures with thin p-type cladding or confinement layers have been developed to simplify the fabrication of distributed feedback and ridge waveguide lasers. The thin p-type confinement or ‘p-clad’ layer (approximately 0.3 μm compared with the standard thickness of approximately 1–1.5 μm) allows the optical mode to have sufficient amplitude at the top surface of the structure to achieve the desired grating coupling by relatively shallow etching at the top surface and without requiring regrowth, which is especially difficult for Al-containing materials. For typical ridge waveguide diode lasers with injection stripe widths of about 2–4 micrometers (μm), the shallowness of the etch significantly reduces underetch. This improves the series resistance of the device. Moreover, the tolerance on the thickness of the material that remains above the active layer and outside the ridge area after etching is very tight, typically only a few tens of nanometres. With a conventional diode laser structure, this typically corresponds to an etch depth greater than about 1 μm, and it is very difficult to etch the thick cladding layer with sufficient uniformity to meet these tolerances. In contrast, a thin p-clad laser structure requires an etch depth of only about 0.3 μm, significantly reducing variations in waveguiding due to variations in etch depth, and thus improving the kink-free operation of the laser.
However, prior art thin p-clad structures have been characterised by large optical losses of α≈10 per centimeter (cm−1) or more. Additionally, the transverse (vertical) divergence generally exceeds about 40° due to a need to confine the optical field to reduce absorption in the metal and p++ GaAs contact layers. A structure with low loss (e.g., below about 3 cm−1) and reduced divergence is desirable for efficient operation of pump lasers for erbium-doped fiber amplifiers, the main application of 980 nanometer diode lasers. It is desired, therefore, to provide a thin clad diode laser that alleviates one or more of the above difficulties, or at least a useful alternative to existing diode lasers.